The present invention relates to an expansion device employed in a freezing cycle in an air-conditioning system for vehicles and, more specifically, it relates to an expansion device having a mechanism for preventing an abnormality from occurring with regard to the high-pressure in a freezing cycle which uses carbon dioxide as a coolant.
In the freezing cycle disclosed in Japanese Unexamined Patent Publication No. H 7-25231 representing a typical example of the prior art, which uses a freon coolant and comprises, at least, a compressor that compresses the coolant, a condenser connected in series to the compressor, an expansion valve and an evaporator, an auxiliary coolant passage is provided in parallel to the expansion valve, a valve for opening/closing the auxiliary coolant passage is provided and the auxiliary coolant passage is opened if the low-pressure reaches a level equal to or lower than a specific value.
Thus, a reduction in the low-pressure is prevented by allowing the high-pressure to bypass the expansion valve to flow directly into the low-pressure side and the pressure on the outlet side of the compressor and the compression temperature are prevented from rising in this example.
In addition, there are freezing cycles provided with a low-pressure cutoff switch to turn off the cycle based upon a decision that the quantity of coolant is insufficient or that the temperature of the external air is low and, therefore, the load is low if the high-pressure reaches a level equal to or lower than a specific value.
Other safety mechanisms that may be provided in freezing cycles include a mechanism through which the operation of the compressor is stopped if the high-pressure reaches a level equal to or higher than a specific value, a mechanism through which the operation of the compressor is stopped if the compressor outlet temperature reaches a level equal to or higher than a specific value, a mechanism through which the high-pressure coolant is released into the atmosphere if the high-pressure reaches a level equal to or higher than a specific value and a fusible plug that allows the coolant to be released into the air if the coolant temperature reaches a specific value.
While concentrated efforts have been made to research into alternatives to freon, such as carbon dioxide (CO2), to be used as coolant in a freezing cycle in air conditioning systems for vehicles in addressing the global environment issue, carbon dioxide has a low critical point of approximately 31.1xc2x0 C. and, thus, a freezing cycle in which carbon dioxide is used as the coolant constitutes a super-critical cycle crossing over the critical point, resulting in the high-pressure therein reaching a level as high as 10 times the high-pressure in a freezing cycle using a freon coolant. When designing a heat exchanger and the like by taking into consideration the relevant safety factors, it is even more crucial to include a safety device for cycle protection in the freezing cycle in which an alternative coolant is used than in a freezing cycle (existing cycle) in the prior art that uses a freon coolant, since the high-pressure rises to a level close to the pressure withstanding limit of the aluminum material.
In more specific terms, the super-critical cycle described above, in which the normal operating pressure on the high-pressure side is approximately 10xcx9c15 MPa and the coolant does not cross over the critical point to become condensed, achieves characteristics whereby the high-pressure responds more sensitively to a load fluctuation compared to a cycle using a freon coolant in which the high-pressure side coolant becomes condensed. Accordingly, it has been confirmed that the likelihood of the high-pressure in a super-critical cycle reaching a level near the maximum normal operating pressure is far greater than the likelihood in the existing cycle. Thus, it becomes necessary to prevent an excessive rise in the high-pressure by responding to any increase in the high-pressure with a high degree of sensitivity.
In addition, in the super-critical cycle in which the critical point of the coolant is low, the balance pressure between the high-pressure side and the low-pressure side within the super-critical cycle increases as high as approximately 10 MPa if the cycle is left outdoors when the temperature is very high. Thus, it is necessary to protect the devices on the low-pressure side from such an increase in the balance pressure.
While a special safety means may be provided individually on the high-pressure line and the low-pressure line, it is more desirable to provide an entire safety mechanism at one component, e.g., the expansion device, from the viewpoint of achieving simplification in the structure of the freezing cycle and also simplification of the work process.
Accordingly, an object of the present invention is to provide an expansion device employed in a freezing cycle that uses carbon dioxide as the coolant, which is capable of preventing an abnormal increase in the high-pressure and responding quickly to abnormal rises in the high-pressure and the low-pressure.
In order to achieve the object described above, the expansion device according to the present invention, which is employed in a freezing cycle that uses carbon dioxide as a coolant and constitutes the freezing cycle together with, at least, a compressor that compresses the coolant to achieve a pressure in the super-critical range, a radiator that cools the compressed coolant and an evaporator that evaporates the coolant, having a valve housing, a high-pressure passage formed inside the valve housing through which the high-pressure coolant discharged from the radiator flows in, a restrictor valve mechanism provided on the downstream-most side of the high-pressure passage that reduces the pressure of the high-pressure coolant and a low-pressure passage through which the coolant, the pressure of which has been lowered by the restrictor valve mechanism, flows out to the evaporator, is further provided with a high-pressure space formed inside the valve housing and communicating with the high-pressure passage, a relief hole that communicates between the high-pressure space and the low-pressure passage, a means for displacement provided inside the high-pressure space that becomes displaced in correspondence to the pressure inside the high-pressure space, a rod passing through the relief hole that links the front end of the means for displacement and a valve element of the restrictor valve mechanism and a safety valve mechanism provided at the rod that is constituted of a first portion which has a diameter approximately equal to the diameter of the relief hole and blocks the relief hole and a second portion located between the first portion and the valve element of the restrictor valve mechanism, which has a diameter smaller than the diameter of the relief hole and opens the passage through the relief hole, and communicates between the high-pressure space and the low-pressure passage if the pressure inside the high-pressure space reaches a level equal to or higher than a first specific pressure.
As a result, the means for displacement that becomes displaced in correspondence with the level of the, high-pressure, displaces the rod provided with the safety valve mechanism and, if the high-pressure reaches a level equal to or higher than the first specific pressure (the limit of the normal operating pressure), the first portion of the safety valve mechanism blocking the relief hole becomes disengaged from the relief hole to be replaced by the second portion which allows passage through the relief hole, thereby leaking the coolant in the high-pressure space into the low-pressure passage to prevent an increase in the high-pressure. It is to be noted that the first specific pressure may be, for instance, 15 MPa.
In addition, it is desirable to provide a low-pressure side rupture disk that becomes ruptured if the low-pressure reaches a second specific pressure lower than the first specific pressure to allow the low-pressure passage to communicate with the atmosphere in the low-pressure passage. By providing such a rupture disk, the low-pressure side rupture disk, which becomes ruptured if the low-pressure rises to an abnormally high-level equal to or higher than the second specific pressure for any reason including the cycle having been left outdoors where the temperature is extremely high, and allows the low-pressure passage to become communicated with the atmosphere in such an event to release the coolant, thereby preventing any damage to the air conditioning devices provided on the low-pressure side is prevented. It is to be noted that the second specific pressure may be, for instance, 10 MPa.
Furthermore, it is desirable to provide a high-pressure side rupture disk that becomes ruptured if the high-pressure reaches a level equal to or higher than a third specific pressure which is higher than the first specific pressure to allow the high-pressure passage to become communicated with the atmosphere, in the high-pressure passage. By providing the high-pressure side rupture disk which becomes ruptured if the high-pressure reaches a level equal to or higher than the third specific pressure due to an abnormality, the high-pressure passage is allowed to communicate with the atmosphere to release the coolant in such an event, thereby preventing any damage to the air conditioning devices provided on the high-pressure side and the low-pressure side. It is to be noted that the third specific pressure may be, for instance, 17.5 MPa.
Moreover, it is desirable to constitute the means for displacement with a bellows that expands and contracts corresponding to the level of the high pressure. While the diaphragm may be used to constitute the means for displacement instead of a bellows, a bellows which is capable of assuring a sufficient displacement quantity will be preferable.